The invention relates to making a protective coating on a superalloy metal substrate.
The field of application of the invention is making parts that are capable of retaining mechanical strength at high temperatures, in particular gas turbine parts, such as turbine blades, in particular for turbojets.
In order to improve performance, and in particular efficiency, it is desirable to cause gas turbines to operate at temperatures that are as high as possible.
In order to make parts for the hot portions, it is common practice to use superalloys. These alloys usually comprise nickel as the main constituent and additional elements generally selected from chromium, cobalt, aluminum, molybdenum, titanium, tantalum, and many others.
An increase in operating temperature is made possible by providing the metal substrate of such parts with a protective coating forming a thermal barrier.
It is known to make a protective coating comprising an ceramic outer layer and a bonding underlayer of metal, in particular a bonding underlayer containing aluminum and at least one other metal, such as platinum.
The bonding underlayer interposed between the superalloy metal substrate and the ceramic outer layer performs the following functions:                it enables a film of alumina to form and to persist at its surface, thereby enhancing bonding with the ceramic outer layer;        it protects the substrate from corrosion by oxidation by the oxygen in the ambient medium that manages to pass through the outer ceramic layer; and        it constitutes a diffusion barrier against certain elements of the metal substrate which would otherwise contaminate the film of alumina, and would consequently affect the interface between the bonding underlayer and the outer ceramic layer, thereby affecting the adhesion thereof.        
Including reactive elements such as yttrium, cerium, hafnium, or lanthanides in the bonding underlayer reinforces its diffusion barrier function and enhances the persistence of the “adhesive” film of alumina.
It is well known to form a bonding underlayer of the MCrAlY type (where M is a metal such as Fe, Ni, Co) by a method such as plasma projection, without inducing a reaction with the substrate, the adhesion of the bonding underlayer on the substrate being of a mechanical kind. Reference can be made, for example, to U.S. Pat. Nos. 4,055,704 and 5,824,423. Nevertheless, in order to obtain an under-layer that is thermally stable, it is necessary to give it relatively large thickness, typically at least a thickness lying in the range 50 micrometers (μm) to 100 μm, and such thickness is penalizing in terms of weight.
Other known methods consist in making a bonding underlayer out of an intermetallic compound, which can be of smaller thickness because of its thermal stability. An intermetallic compound comprising aluminum and platinum has been found to have good properties.
Thus, U.S. Pat. Nos. 5,716,720 and 5,856,027 describe a method consisting in electroplating a layer of platinum on a substrate made of a nickel-based superalloy, and subsequently in performing vapor aluminization at a temperature greater than 1000° C. Nickel coming from the substrate diffuses into the bonding underlayer. An alumina film is formed by heat treatment at the surface of the bonding underlayer prior to forming a ceramic outer layer, e.g. of yttria stabilised zirconia obtained by physical vapor deposition (PVD).
A reactive element may be introduced into the bonding underlayer during the vapor aluminization step. In its outer portion surmounting a diffusion zone in the vicinity of the substrate, the bonding underlayer presents an intermediate phase comprising 18% to 28% by weight aluminum, 50% to 60% by weight nickel, and 8% to 35% by weight platinum, corresponding to a β-type solid solution phase in the binary nickel-aluminum phase diagram (β-NiAl).
U.S. Pat. No. 5,238,752 describes another method consisting in forming on a superalloy substrate a bonding underlayer made of an intermetallic compound, in particular a compound of aluminum and platinum. The bonding under-layer is made by pack cementation at a temperature higher than 985° C. and at a thickness greater than 25 μm. An alumina film is formed by oxidation at the surface of the bonding underlayer prior to forming a ceramic outer layer, e.g. of yttria stabilised zirconia by physical vapor deposition.
European patent application EP 0 985 744 describes yet another method comprising forming a layer of platinum on a nickel-based superalloy substrate by electroplating or by chemical vapor deposition, and depositing a layer of aluminum which is made from a gaseous halide and which diffuses into the layer of platinum. Sulfur is removed after each deposit by heat treatment at a temperature higher than 1050° C. with surface descaling so as to eliminate sulfur which is harmful to adhesion of the alumina film developed on the surface of the resulting bonding underlayer.
US patent application No. US 2002/0037220 discloses a method in which the bonding underlayer is formed by physical vapor deposition of a plurality of individual layers alternately of aluminum and of a metal from the platinum group, and by exothermal reaction between the metals of the layers formed. By using a physical vapor deposition method, the temperature of the substrate is relatively low, and remains at a value well below that from which the elements of the substrate are liable to diffuse into the deposit being formed.